This invention relates to active electrostatic seals and electrostatic vacuum pumps and, more particularly, to devices and methods wherein gas is transported between closely spaced or contacting surfaces of arbitrary shape. The devices and methods of the invention may be utilized in electrostatic wafer clamps for retaining a coolant gas, in face seals and in shaft seals, but are not limited to such uses.
In the fabrication of integrated circuits, a number of well-established processes involve the application of ion beams to semiconductor wafers in vacuum. These processes include, for example, ion implantation, ion beam milling and reactive ion etching. In each instance, a beam of ions is generated in a source and is directed with varying degrees of acceleration toward a target wafer. Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity.
The wafer mounting site is a critical part of an ion implantation system. The wafer mounting site is required to firmly clamp a semiconductor wafer in a fixed position for ion implantation and, in most cases, to provide cooling of the wafer. In addition, means must be provided for exchanging wafers after completion of ion implantation. In commercial semiconductor processing, a major objective is to achieve a high throughput in terms of wafers processed per unit time. One way to achieve high throughput is to use a high current ion beam so that the implantation process is completed in a shorter time. However, large amounts of heat are likely to be generated by the high current ion beam. The heat can result in uncontrolled diffusion of impurities beyond prescribed limits in the wafer and in degradation of patterned photoresist layers. Accordingly, it is usually necessary to provide wafer cooling in order to limit the maximum wafer temperature to about 100xc2x0 C., and limiting the maximum wafer temperature to less than 100xc2x0 C. may be required in the future.
A number of techniques for clamping a semiconductor wafer at the target mounting site are known in the art. One known technique involves the use of electrostatic forces. A dielectric layer is positioned between a semiconductor wafer and a conductive support plate. A voltage is applied between the semiconductor wafer and the support plate, and the wafer is clamped against the dielectric layer by electrostatic forces. An electrostatic wafer clamp is disclosed by G. A. Wardly in xe2x80x9cElectrostatic Wafer Chuck for Electron Beam Microfabricationxe2x80x9d, Rev. Sci. Instrum., Vol. 44, No. 10, October 1972, pp. 1506-1509 and in U.S. Pat. No. 3,993,509 issued Nov. 23, 1976 to McGinty. Electrostatic wafer clamp arrangements which utilize a thermally-conductive material to remove heat from the wafer are disclosed in U.S. Pat. No. 4,502,094, issued Feb. 26, 1985 to Lewin et al., U.S. Pat. No. 4,665,463, issued May 12, 1987 to Ward et al., and U.S. Pat. No. 4,184,188, issued Jan. 15, 1980 to Briglia. The Briglia patent discloses a support plate having layers of thermally-conductive, electrically-insulative RTV silicone. Electrostatic wafer clamps are also disclosed in U.S. Pat. No. 4,480,284, issued Oct. 30, 1984 to Tojo et al., U.S. Pat. No. 4,554,611, issued Nov. 19, 1985 to Lewin, U.S. Pat. No. 4,724,510, issued Feb. 9, 1988 to Wicker et al. and U.S. Pat. No. 4,412,133, issued Oct. 25, 1983 to Eckes et al.
An electrostatic wafer clamp that provides highly satisfactory performance is disclosed in U.S. Pat. No. 4,452,177, issued Sep. 19, 1995 to Frutiger. A six-phase electrostatic wafer clamp includes a platen having six sector-shaped electrodes. Voltages with six different phases are applied to the electrodes, with the voltages applied to electrodes on opposite sides of the platen being one-half cycle out of phase. The applied voltages are preferably bipolar square waves.
As indicated above, wafer cooling is typically required during ion implantation. The technique of gas conduction has been utilized for wafer cooling in vacuum. A coolant gas, introduced into a region between the semiconductor wafer and the clamping surface, provides thermal coupling between the wafer and a heat sink. Gas conduction in an electrostatic wafer clamp is disclosed in the aforementioned U.S. Pat. No. 5,452,177.
Wafer clamps which employ gas conduction cooling typically employ means for retaining the coolant gas in the region between the wafer and the clamping surface and thereby limiting leakage of the gas into the vacuum chamber. Such leakage reduces cooling effectiveness and contaminates the vacuum chamber.
Several prior art techniques have been utilized for retaining the coolant gas. One approach uses a perimeter seal, such as an O-ring or a lip seal, at the perimeter of the clamping surface, as disclosed for example in the aforementioned U.S. Pat. No. 5,452,177. The sealing surface comes into contact with the perimeter of the wafer, sealing against the wafer. However, the perimeter seal can easily become damaged, since it is exposed on the clamping surface. The perimeter seal may lose effectiveness easily, becoming contaminated over time with the particulates that are inevitable in process chambers. Particles may be generated by the seal rubbing against the wafer. The rough back side of the silicon wafer itself may compromise the seal. Even when the seal is not compromised, an elastomeric seal is permeable to hydrogen, helium and the lighter gases. Further, an elastomeric seal suffers from compression set and degradation due to harsh processing environments such as radiation and/or severe chemicals.
Another approach to retaining the coolant gas utilizes an area seal, where the wafer is electrostatically clamped against a polished platen surface, providing a minimal clearance between the platen and the wafer, and limiting gas leakage. An area seal produced by the electrostatic clamping of a wafer against a flat and finely polished clamping surface is more resistant to damage than the perimeter seal. However, the area seal may be somewhat more susceptible to leakage due to trapped particles which increase the space between the wafer and the clamping surface. This drawback may be alleviated somewhat by the flexibility of the wafer, and the edge of the wafer may seal around the perimeter despite particles trapped toward the center. However, the increased gas pressure required for adequate cooling requires increased clamping voltage to maintain the wafer clamped against the clamping surface. Typically, as the coolant gas pressure increases, the leak rate also increases.
Another technique for limiting coolant gas leakage into the vacuum chamber utilizes an annular groove around the periphery of the clamping surface. The groove is connected to a vacuum pump, and the coolant gas is removed before it leaks into the vacuum chamber. See, for example, U.S. Pat. No. 4,603,466, issued Aug. 5, 1986 to Morley. This approach has the disadvantages of reduced clamping force in the case of an electrostatic wafer clamp and reduced cooling in the region of the annular groove.
The above-identified problem of gas leakage from the periphery of an electrostatic wafer clamp is an example of a more general sealing problem which involves the leakage of gas between two closely spaced or contacting surfaces of arbitrary shape. Another example of the sealing problem occurs in a shaft seal wherein a shaft extends through a wall from a region of higher pressure to a region of lower pressure. The surfaces cannot be permanently sealed, such as with an adhesive, because of relative movement between the surfaces. In the case of the electrostatic wafer clamp, the wafer is removed after processing. In the case of the shaft seal, the shaft is movable relative to the seal in which it is mounted.
Accordingly, there is a need for improved techniques for limiting leakage of a gas between closely spaced or contacting surfaces.
According to a first aspect of the invention, an electrostatic device comprises a conductive element and a dielectric element each having a surface, the surfaces of the dielectric element and the conductive element being closely-spaced or contacting, one of the conductive element and the dielectric element being flexible, a plurality of electrodes positioned adjacent to and electrically isolated from the surface of the dielectric element, and a voltage source for applying voltages to the electrodes for transporting a gas located between the surfaces of the conductive element and the dielectric element. The voltages applied to the electrodes may produce in the flexible element a moving wave which transports the gas.
The electrostatic device may function as an electrostatic seal between the conductive element and the dielectric element or as an electrostatic vacuum pump.
In one embodiment, the surfaces of the dielectric element and the conductive element are substantially planar. Each of the electrodes may comprise a closed loop of arbitrary shape. In one example, the electrodes comprise concentric rings.
In another embodiment, the surfaces of the dielectric element and the conductive element are substantially cylindrical. The conductive element may comprise a shaft and the dielectric element may be flexible. The electrodes may comprise axially-spaced rings, thereby providing an electrostatic shaft seal.
The voltage source may generate voltages that each include attractive voltage segments and non-attractive voltage segments in a repeating sequence. The voltages are phased such that the attractive voltage segments and the non-attractive voltage segments move from electrode to electrode and define a direction of gas transport.
In one embodiment, the surfaces of the dielectric element and the conductive element have a periphery, and the electrodes are located at or near the periphery of the surfaces for transporting gas away from the periphery and thereby limiting leakage of the gas at the periphery of the surfaces.
According to another aspect of the invention, an electrostatic device comprises a conductive element and a dielectric element each having a surface, the surfaces of the dielectric element and the conductive element being closely-spaced or contacting, one of the conductive element and the dielectric element being flexible, and a plurality of electrodes positioned adjacent to and electrically isolated from the surface of the dielectric element for transporting a gas located between the surfaces of the dielectric element and the conductive element in response to voltages applied to the electrodes.
According to a further aspect of the invention, an electrostatic device is provided for sealing to a surface of a conductive workpiece. The electrostatic device comprises a dielectric element having a surface, the surfaces of the workpiece and the dielectric element being closely-spaced or contacting when the device is in use, one of the workpiece and the dielectric element being flexible, a plurality of electrodes positioned adjacent to and electrically isolated from the surface of the dielectric element, and a voltage source for applying voltages to the electrodes for transporting a gas located between the surfaces of the workpiece and the dielectric element.
According to yet another aspect of the invention, a method is provided for transporting a gas. The method comprises the steps of providing a conductive element and a dielectric element having surfaces that are closely-spaced or contacting, one of the conductive element and the dielectric element being flexible, positioning a plurality of electrodes adjacent to and electrically isolated from the surface of the dielectric element and applying voltages to the electrodes for transporting a gas located between the surfaces of the conductive element and the dielectric element.